In southern Bangladesh's muddy refugee camps, Rohingya Muslims who have fled what the UN terms "textbook" ethnic cleansing are fighting each other for scarce space and basic necessities, an aid worker has told the ABC.

"I have no words to describe what I'm seeing out there," said International Federation of the Red Cross spokeswoman, Corinne Ambler of conditions in the impromptu refugee settlements currently spreading ever deeper into the forests near Bangladesh's border with Myanmar.

"Wall-to-wall human suffering, that's what it is.

"There's little clean water, we've seen people fighting over money, over food, its undignified, and its catastrophic really."

Myanmar's military has for several weeks now been conducting operations it says are aimed at Muslim terrorists in Myanmar's western Rakhine state.

The brutal campaign has this week been labelled "ethnic cleansing" by the UN.

In less than a month it has prompted an unprecedented number, nearly 400,000, to escape into Bangladesh, overwhelming aid workers and leaving many to fend for themselves.

Ms Ambler says desperate and destitute, new arrivals are battling each other for the basics of life.

"Well-meaning people are flinging clothes and food and cash from the top of trucks and its just not the way to distribute aid," she said.

"Crowds of people, children, men and women are running after the trucks, grabbing at whatever they can get."

The exodus amounts to nearly 20,000 people a day.

"It's just a stream that's not ending, you know, there's nowhere for them to go, but still they come," Ms Ambler said.

"You think every day that it's not going to get worse, but it does get worse. We're watching truckloads and truckloads of people filing in."

Deepening suffering, endless need

This scale and speed of the Rohingya migration has taken the global community by surprise, and sees them arriving in a poor country without the ability to care for them.

Marixie Mercado, spokeswoman for the UN's child agency, UNICEF said the international response has so far been woefully inadequate.

"Far, far more is needed, not only in funding but also in terms of hands on the ground to help scale up this relief operation," she said.

"The needs are seemingly endless and suffering is deepening," she said, warning that unless they were helped, there was a risk of unrest.

"There is tension rising in both the refugee camps and in the informal settlements."

Ms Mercado said more than half the refugees (240,000) were children, who needed basic care for a shot at survival.